Microelectronics, microsensors, and microelectromechanical systems (MEMS) typically utilize energy sources located off-chip. Integrating microscale energy storage on-chip with microdevices is essential for achieving autonomous devices. Electrical energy for microdevices can be provided by either capacitors or batteries. Capacitors can charge and discharge very quickly, but inherently contain very little energy. Traditional batteries contain large amounts of energy, but cannot charge or discharge quickly. Other power sources, such as fuel cells, are practical for larger systems, but are not easily miniaturized.
Batteries are limited by their maximum power density/discharge rate because of slow kinetics related to ion and electron transport. Reducing the characteristic ion and electron diffusion lengths within the active battery material has proven to be successful in increasing power densities and discharge rates; however, this has also resulted in a substantial decrease in energy density. Miniature batteries have been developed to power cm2 sized devices and microelectronics, but they have not seen widespread adoption due to limits in their energy and power capabilities. Thin-film lithium ion batteries, for example, have high power densities due to thin active material layers (<1 μm), but the total power and energy provided is generally not sufficient to meet the demands of micro devices due to the two-dimensional architecture inherent to thin films. Building into the third dimension—e.g., making thicker active material layers—can boost the energy density; however, electron and ion diffusion lengths concomitantly increase, thereby reducing power density.
3D bicontinuous porous electrodes can enable rapid charge and discharge for lithium ion batteries because of their shortened pathways for both liquid-phase and solid-phase ion diffusions. Recently, this type of porous electrode has been integrated into an interdigitated configuration for microbatteries that can exhibit two times greater energy density and two thousand times greater power density compared to previous structures. Such microbatteries may be realized by independently electroplating anode and cathode active materials on interdigitated 3D porous nickel scaffolds formed from a colloidal template. Although this technology has set a few new records for high power microbatteries, there are several important issues to be addressed. The nickel current collector may grow isotropically instead of vertically during bottom-up deposition, leading to hemispherically shaped electrodes that do not fully utilize the device volume, and thus may not exhibit the maximal energy density. Microbatteries often require tall electrodes (˜100 μm) to achieve high areal energy density. If the electrode width increases simultaneously with height during fabrication, the power performance may be degraded and the areal density limited.